RE: Multiple DSC radio operation
My MMSI number was issued by the FCC as a part of my station license. I don't think the FCC would issue 2 numbers to the same boat. I don't know about a Boat US number, maybe that's what he has.
In BC, Canada, the Canadian CG uses the DSC alarm to alert boaters to an emergency or to severe weather. I have had the alarm go off at least once per summer and sometimes more often.
If both radios have the same MMSI number, all that is required is to shut off, (power down) one radio, and then respond to the alarm on the other.
If the alarm is not an emergency alarm, but is a ringing indicating an incoming call, it goes to the VHF being 'dialed'. Having 2 MMSI numbers on two different radios could cause confusion, but as Ed said, looking at the display should clear up the problem.